How much weight do you have to lose before people notice?

You've dieted and exercised for months, and finally the pounds are falling away. So, when do people start noticing the difference in your face?

Canadian researchers believe they've found out.

"Women and men of average height need to gain or lose about three and a half and four kilograms, or about eight and nine pounds, respectively, for anyone to see it in their face. But they need to lose about twice as much for anyone to find them more attractive," Nicholas Rule, an associate professor and Canada Research Chair in Social Perception and Cognition at the University of Toronto, said in a university news release.

The face "is a robust indicator of one's health," said Rule. "Increased facial adiposity [fat content] is associated with a compromised immune system, poor cardiovascular function, frequent respiratory infections and mortality. So, even a small decrease can improve one's health."

To determine how much weight people need to lose before they appear more attractive to others, the researchers compiled a collection of photos that featured the faces of men and women in their 20s and 30s. The people in the photos didn't wear any jewelry, their hair was pulled back and they had neutral facial expressions.

Each image was duplicated and altered slightly to create a sequence of photos in which the person appeared to gradually gain weight.

The participants compared randomly drawn pairs of faces from each of the sequences and chose the one that looked heavier. Based on their responses, the researchers gauged the weight loss needed before all those dropped pounds become noticeable in a person's face.

Rule's team also found that people needed to lose twice as much weight to be perceived as more attractive by others beyond just "you've lost weight." The average amount of weight loss needed to make the faces in photos more appealing was about 14 pounds for women and 18 pounds for men of average height.

The findings are important because, "when it comes to incentives for weight loss, some people are more motivated to look attractive than to improve their health," study co-author Daniel Re, a postdoctoral fellow at the university, said in the news release.

"The difference between the groups suggests women's facial attractiveness may be more sensitive to changes in weight," she noted. "This just means women attempting to lose weight need to shed slightly fewer pounds than men for people to find them more attractive."

The findings were published recently in the journal Social Psychological and Personality Science.